A semiconductor physical quantity sensor constructed by joining two substrates has been proposed. For example, Japanese Patent Application Publication No. 11-267777 (counterpart U.S. Pat. No. 6,028,332) describes a semiconductor-type yaw rate sensor in which a lower wiring layer is formed on a substrate and another substrate formed with a beam structure for detecting a yaw rate is laid on the lower wiring layer.
Such a yaw rate sensor is manufactured in the following manner. First, a first polycrystalline silicon layer as the lower wiring layer is formed on a first silicon substrate. Next, the first polycrystalline silicon layer is patterned into a wire shape. Then, an insulating layer is formed on the surface of the patterned first polycrystalline silicon layer for electrically insulating the wiring from the beam structure.
Thereafter, a second polycrystalline silicon layer is formed above the insulating layer so as to fill projections and grooves at the surface of the insulating layer. Next, the surface of the second polycrystalline silicon layer is flattened such as by mechanical polishing or the like in order to enhance attachment of a second silicon substrate thereto. Then, the second silicon substrate is attached to the flattened surface of the second polycrystalline silicon layer. Further, the second silicon substrate is thinned by mechanical polishing or the like. Thus, a support substrate is prepared.
Thereafter, the first silicon substrate is thinned by mechanical polishing or the like, and the beam structure is formed in the thinned first silicon substrate by a photolithography and etching process. Further, the insulating film located between a movable portion of the beam structure and the second silicon substrate is removed such that the movable portion is movable relative to a fixed portion of the beam structure. In this way, the semiconductor-type yaw rate sensor is manufactured.
Such a manufacturing process begins preparation of the first silicon substrate in which the beam structure is to be formed, and includes forming of the lower wiring layer on the first silicon substrate and the like and joining the second silicon substrate to the first silicon substrate. Therefore, the process is long, and a layer structure is complicated.
Particularly, since the second silicon substrate is joined to the second polycrystalline silicon layer, which is layered on the insulating layer covering the lower wiring layer of the first silicon substrate, it is necessary to flatten the second polycrystalline silicon layer. Fatness of the surface of the second polycrystalline silicon layer is likely to affect a characteristic of the semiconductor yaw rate sensor. It is difficult stably manufacture the semiconductor-type yaw rate sensors to have stable characteristics.
Such a drawback is not limited to the semiconductor-type yaw rate sensor, but is likely to be caused in semiconductor physical quantity sensors constructed by joining two substrates in the similar manner.